I have a Telstra supplied Netgear CG3100D-2BPAUS model and a Netgear Nighthawk X4 AC2200 Extender. The extender is emtting what is labelled as a 5Ghz extension on preset channel 44 but I know that the CG3100 does not support 5Ghz.
Is that signal really just a duplicate of the 2.4Ghz channel?
Depends on the repeater configuration.
If using the Fast Lane set-up, the repeater does connect to the primary access point using one radio, e.g. the 2.4 GHz - and does offer an access point on e.g. on the 5 GHz only.
In the "normal" configuration the repeater does "bridge" the 2.4 GHz clients to the 2.4 GHz radio on the primary access point, the 5 GHz clients to the 5 GHz radio on the primary access point. In the case where a repeater is not able to connect to one of the bands, the band won't be usable on the extender (unless they changed the implementation recently) even if the 5 GHz signal from the repeater is on air. Give it a try - my guess is that the 5GHz clients won't be able to connect to your router/network/Internet.
PS. There is a dedicated area for WiFi Range Extenders and Nighthawk Mesh